2021 IMO
Revision as of 10:02, 29 January 2023 by Mathhyhyhy (talk | contribs)
For the statement to be true, there must be at least three pairs whose sum is each a perfect square. There must be p,q,r such that p+q = x^2 and q+r = y^2, p+r = z^2.
WLOG n<= p<= q<= r <= 2n ... Equation 1
p = x^2 + z^2 - y^2
q = x^2 + y^2 – z^2
r = y^2 + z^2 – x^2
by equation 1
2n <= x^2 + z^2 – y^2 <= 4n
2n <= x^2 + y^2 – z^2 <= 4n
2n <= y^2 + z^2 – z^2 <= 4n
6n <= x^2 + y^2 + z^2 <= 12n
6n <= 3x^2 <= 12n
2n <= x^2 <= 4n
√(2n) <= x <= 2√n
At this time n >= 100, so
10 * √2 <= x,y,z <= 20
15 <= x,y,z <= 20
where
2|x^2 + y^2 – z^2 2|x^2 + z^2 – y^2 2|y^2 + z^2 – z^2 x = 16, y = 18, z = 20 fits perfectly
therefore the proposition is true